fix(setup_assistant): use field defaults when controller params left blank

[darshmenon]

Summary

Fixes #3434.

When a user opens the MoveIt Setup Assistant controller edit widget and leaves the action_ns or default additional-parameter fields blank (e.g. by clicking through without typing), getAdditionalParameters() stored the empty string directly into the parameters map. The resulting ros2_controllers.yaml then contained:

- name: my_arm_controller
  type: joint_trajectory_controller/JointTrajectoryController
  action_ns: ""
  default: ""
  joints: [...]

These empty-string values are semantically wrong and cause downstream parsing errors when the YAML is loaded by ros2_control.

Root Cause

typeChanged() (called on combo-box selection) already calls getDefaultValue(controller_type) to pre-fill the fields. But getAdditionalParameters() (called when the user saves) never did the same fallback — it emitted whatever was in the text box, including empty strings.

Fix

In getAdditionalParameters():

1. Call getControllerType() once (already a public method on the same class).
2. If a field's trimmed text is empty, fall back to getDefaultValue(controller_type).
3. If the value is still empty after the fallback, skip the key entirely rather than emitting an empty-string entry.

std::map<std::string, std::string> ControllerEditWidget::getAdditionalParameters()
{
  std::map<std::string, std::string> parameters;
  std::string controller_type = getControllerType();
  for (unsigned int i = 0; i < additional_fields_.size(); i++)
  {
    std::string key = additional_fields_[i]->parameter_name_;
    std::string value = additional_fields_inputs_[i]->text().trimmed().toStdString();
    if (value.empty())
    {
      value = additional_fields_[i]->getDefaultValue(controller_type);
    }
    if (!value.empty())
    {
      parameters[key] = value;
    }
  }
  return parameters;
}

Test Plan

• Open MoveIt Setup Assistant, add a joint_trajectory_controller/JointTrajectoryController controller, leave action_ns and default blank, export config — verify ros2_controllers.yaml now contains action_ns: follow_joint_trajectory and default: "true" rather than empty strings.
• Same test with GripperCommand controller — verify action_ns: gripper_cmd.
• Add a controller, manually type a custom action_ns value — verify the explicit value is preserved.
• Load an existing YAML with populated fields — verify round-trip is lossless.